Poly(vinyl alcohol) is a semi-crystalline, water soluble polymer that is used in many applications, such as textiles sizing, paper coating and adhesives manufacturing. Poly(vinyl alcohol) can also be manufactured as fibers, and the fibers can be produced with a wide range of properties. By using poly(vinyl alcohol) with a high degree of hydrolysis and crystallinity, very strong fibers that are water resistant can be produced. On the other hand, by slightly lowering the degree of hydrolysis, fibers that are still hot water soluble can be produced. Although these fibers have a wide range of properties, they are difficult to manufacture. The two most common preparation techniques are the wet spinning and dry spinning processes.
In the wet spinning process, poly(vinyl alcohol) is washed with cold water and dissolved in hot water. Because of the high viscosities of these solutions, the concentration of poly(vinyl alcohol) is limited to 14-17 wt %. The solutions are then filtered, deaerated, and extruded through fine hofles of a spinnerette into a coagulation bath. The resulting fiber is subjected to drawing in a hot aqueous solution of sodium sulfate, dried, drawn in hot air, and subjected to heat treatment. Because low polymer concentrations are used in this process, all of the excess water must be flashed to produce a dry fiber. This process is very energy intensive.
In the dry spinning process, more concentrated poly(vinyl alcohol) solutions of 25 to 50 wt % can be prepared in batch or a continuous blender. This solution is extruded through a spinnerette and coagulated in a hot cell. The coagulated, dried fiber is subsequently drawn in hot air and then subjected to heat treatment.
Other methods for preparing poly(vinyl alcohol) solutions include pre-blending the poly(vinyl alcohol) and water (without dissolving) as a feed to an extruder, mixing low concentration solutions of poly(vinyl alcohol) with dry powders, and using a twin screw extruder to perform the dissolution step. Representative examples of methods used in preparing poly(vinyl alcohol) solutions are described below:
GB 1,033,395 (1966) discloses the preparation of polyvinyl alcohol solutions containing 35 to 55% polyvinyl alcohol by feeding a mixture of polyvinyl alcohol and water, containing at least 55% polyvinyl alcohol, into a continuous mixer, such as a screw extruder, having one or more intermediate entry ports in the mixer. The mixture is diluted with water or an aqueous solution of polyvinyl alcohol in which the concentration of polyvinyl alcohol is lower that that of the mixture, by introducing it through the intermediate entry ports. The final concentration of polyvinyl alcohol in the mixture is between 35 and 55%.
JP 51-146154 (1976) discloses a method of melt-extruding polyvinyl alcohol powder containing 60% or less water using a screw extruder.
JP 56-86936 (1981) discloses a method of producing a concentrated aqueous poly(vinyl alcohol) solution in which polyvinyl alcohol is poured from the input port installed at the base of a high-shear, engaging type biaxial extruder, water or an aqueous polyvinyl alcohol solution is charged from an injection port downstream from the input port, under low pressure, and hot-melt kneading is performed to form a uniform polyvinyl alcohol aqueous solution having a polyvinyl alcohol content of at least 50 wt %.
NL 8500428 (1986) (abstract; Derwent Accession Number 86-135721) discloses a continuous method for preparing homogeneous solutions of high molecular weight polymers, such as poly(vinyl alcohol) or polyacrylonitrile, by suspending the finely divided polymer in a solvent and mixing and kneading the suspension in a twin-screw, co-rotating extruder.